The impending emergence of the plutonium recycle nuclear fuel economy as an important element in the world energy supply has made the solution of nuclear materials control problems very important. Many thousands of persons will have daily access to plutonium (Pu) material which could be used for terrorist activities and, as a consequence, the diversion of such material is a major risk factor to be confronted by the nuclear industry.
Aside from strict inventory control, the most reliable method of preventing the diversion of fissionable materials is the detection of the natural radioactivity of such materials at plant exit points. Even inventory control requires the detection of plutonium in the assay of waste material. Techniques involving induced activation, while quite effective, are expensive and pose unacceptable health hazards where personnel inspection is involved.
All fissionable isotopes emit gamma rays which are easily detected, and thus a gamma ray detector provides some measure of control. It does, however, suffer from two major disadvantages:
1. The predominant gamma radiation from the most important fissionable isotopes is low in energy, and hence detection can be prevented by a rather modest amount of shielding.
2. A fuel reprocessing plant will necessarily have to deal with large quantities of nonfissile gamma ray emitting isotopes. Thus the passage of fissionable materials may be effectively masked by the signals arising from other radioactive materials.
Fortunately, the plutonium produced under the high burnup conditions typical in a commercial nuclear power plant contains appreciable quantities of the heavier plutonium isotopes, particularly .sup.240 Pu and .sup.242 Pu, which are appreciable neutron sources via spontaneous fission decay. Most of the uranium isotopes of interest have very small spontaneous fission decay branching ratios, thereby producing a rather low neutron flux unless the material is in the form of a light element compound such as an oxide or fluoride. Such compounds produce neutrons via the (.alpha.,n) reaction. As a high neutron flux is largely unique to the material of interest, it provides the most feasible method for detection of diversion by personnel.
Prevention of detection by shielding of the material is not feasible because the emitted neutrons are energetic and hence must be moderated in energy by a large quantity of hydrogenous material before they can be absorbed. Such a large shield would be an obvious indication of diversion.
A neutron detection system, a portal monitor for detecting neutrons emanated by special nuclear materials, has been described by Fehlau and Eaton in their paper published in Measurement Technology for Safeguards Material Controls (U.S. National Bureau of Standards Special Publication No. 582, 1980), p. 365, entitled "Passive Nuclear Material Detection in a Personnel Portal." As there described, a booth is coated with polyethylene material. Neutrons born in the booth enter the polyethylene, which moderates and reflects some of the neutrons. Arrays of 12 detectors in the booth then detect the neutrons with about 5 percent efficiency. The detectors themselves are .sup.3 He proportional counters each having a detection area of about 1.5 ft..sup.2.
Allemand, et. al., in U.S. Pat. No. 3,984,691, teach the arrangement of small neutron counters in an array to cover a large area. Such arrays using conventional counters are expensive. The probable cost of a counter arrangement using .sup.3 He counters or similar conventional counters is in the vicinity of five to ten thousand dollars.
Cost is a very important consideration in the design of neutron counters for monitoring radioactive materials, as emphasized by the Department of Energy speaker at the 1978 Conference on Analytical Methods for Safeguards and Accountability Measurements of Special Nuclear Materials (U.S. National Bureau of Standards Special Publication 528, 1978), p. 80.
Counters more efficient than conventional .sup.3 He or .sup.10 BF.sub.3 counters have been investigated by C. A. Young in Naval Ocean Systems Center Technical Note 661, Efficiency of a Multi-layer .sup.6 Li Foil Neutron Detector (March 1979). .sup.6 Li permits the efficient detection of neutrons using very thin foils of .sup.6 Li. When a .sup.6 Li nucleus absorbs a slow neutron, it undergoes the reaction: EQU .sup.6 Li(n,.alpha.).sup.3 H+4.78 MeV
The large amount of energy released as kinetic energy in the reaction products allows at least one of them to escape from relatively thick foils into a surrounding gas volume where an ionization track is produced. The electrons resulting from such ionization may be detected on counting wires in a conventional manner. In Young's apparatus the lithium metal foils are mounted between wire arrays which are stretched on fiberglass-reinforced epoxy-board frames. Young's counters have an area of approximately 0.25 ft..sup.2.
A further disadvantage of neutron detectors used heretofore results from the contamination of the gases used in the gas proportional counters. Absorption of neutrons in the counter results in material being released from the inner surfaces of the counter into the proportional counter gas. The gas is poisoned by the formation of ions in the counter during normal operation, which liberates impurities on the counter walls. Periods of guaranteed operation are typically of the order of one year, which may be a serious constraint in a detector designed for continuous operation at a critical control point of a security system.